Hydroida.- 



-RADIATA- 



-ASTEUOIDA. 



3'J9 



of vesicular granules held together bj- a semi-transparent, 

 glairy gelatine. They are very contractile, and can 

 change their form at pleasure. The tentacula sur- 

 rounding the mouth are generally numerous, and are 

 always simple and tilifonii, or tapering a little towards 

 the extremity, and have their surface roughened more 

 or less with granules arranged in an imperfectly verti- 

 cillate fashion. These granules are possessed of a 

 peculiar apparatus adapted for paralyzing and killing 

 the animalcules the Polype feeds upon. They are 

 similar in form and function to what in the Acakphie 

 are called the nettling organs. In the centre of these 

 tentacula is placed the mouth, which is very dilatable, 

 and leads by a short passage into the stomach. 



The Anthozoa are propagated by buds or gemmules, 

 and by eggs ; the former extending their individual 

 life, and the latter multiplying and continuing the 

 species. The bud is a shoot from the body, and is 

 identical in structure with the part of the parent whence 

 it pullulates. The eggs are of several kinds. One sort 

 produces young, which, when first born, are very unlike 

 their parents, and resemble minute Meduste. They 

 are free and unattached, swimming at large in the 

 water, and possess a higher organization than the adult 

 animal. They have senses to guide, and muscles to 

 move the body to and fro ; and it is not until they have 

 undergone a series of changes or metamorphoses, that 

 they become staid, their muscles and nerves disappear, 

 and they fall down into an inferior order. Another 

 sort produces young which undergo no change in their 

 development amounting to a metamorphosis. As soon 

 as they are extruded from the ovisac they settle, root, 

 and glide insensibly into the resemblance of the parent 

 species. A third set are in the form of ciliated animal- 

 cules, which have freedom of motion, and swim in the 

 water as if guided by volition and sense, whirling on 

 their axis, and stopping occasionally as if in search of 

 a situation on wliich to fix themselves. After a short 

 life of this sort they obtain a proper site for their per- 

 manent stay and future growth, and begin to shoot up 

 into those beautiful forms peculiar to the species. 

 Many, if not all of the marine Hydroid polyps, are lumi- 

 nous in the dark. 



The Hydroid anthozoa are composed of numerous 

 species, arranged in several families. 



The family Tubulariida:, the species of which have 

 a distinct polypidom, is represented in our plate of 

 Zoophytes, fig. 5, by the exotic species Tuhularia 

 chjtoidea. 



The family Sertulariidce, in which the polypidom is 

 plant-like, horny, variously branched, and tubular, is 

 represented in our Plate 2, fig. 2, by the British species 

 Scrtidaria (Dynamena) pumUa, the Sea-oak coralline 

 of Ellis, and tig. 14 by the exotic species Plumularia 

 secundaria. 



In the family CamparudariidcE, the polypidom is 

 also plant-like and horny, as in the preceding family. 

 It is illustrated in Plate 2, fig. 1, by the "Sea-thread 

 coralline" of Ellis, Laomedia dicfiotoma, a species found 

 in various parts of Great Britain and Ireland. 



The family Hi/droidce, containing the fresh-water 

 polypes, so well and popularly known under the name 

 of Hydra, though placed amongst the Anthozoa by 



most naturalists, is considered by some others as eiui- 

 valent to an order by themselves. 



In the animals belonging to this family, and which 

 consists of a single genus {Hydra), the Polypes are 

 locomotive, single, naked, gelatinous, sub-cylindrical, 

 but very contractile and mutable in form, the mouth 

 encircled with a single series of granulous filiform ten- 

 tacula. The Ilydrre are all natives of fresh water alone. 

 Though usually found attached, they can nevertheless 

 move from place to place, either by gliding with imper- 

 ceptible slowness ou the base, or by stretching out the 

 body and tentacula to the utmost, fixing the latter and 

 then contracting the body towards the point of fixture, 

 loosening at the same time its hold with tlie base. Their 

 usual motion is very slow; but when seizing their prey, 

 they are nimble and active. They are exceedingly 

 voracious, and their long tentacula are spread out 

 in all directions to entrap their victims. Worms and 

 other annelids are killed almost immediately they aie 

 seized, though Entomostracous crustaceans, which are 

 provided with a shell, frequently escape from their 

 grasp unharmed. This has lately been explained by 

 the investigations into the nettling organs which are 

 found in these tentacula, and which have been already 

 mentioned in treating of the Acakphcc. 



Order II.— ANTHOZOA ASTEROIDA {Asteroid 

 Pulypes). 



The Asteroid polypes have the polypidom variable in 

 form, either free or attached, of a fleshy consistence, 

 strengthened with a horny or calcareous axis, enveloped 

 in a gelatinous crust in which the Polypes are immersed. 

 These are compound, living in societies closely united 

 in a single mass by their outer skin ; and the mouth 

 is surrounded with only eight fringed tentacula. The 

 stomach ends in six or eight elongated processes, which 

 are considered as the oviducts. The polype mass is 

 propagated by gemmation ; and as each species emits 

 its buds in a peculiar form, the shape and size of the 

 mass depends upon the manner or preordained fashion 

 in which the buds are evolved. The species, again, 

 are propagated or increased by eggs. These are expelled 

 from the ovisacs into the stomach, and from thence 

 ejected into the sea. They are ciliated, and possess 

 motion, as if apparently actuated by volition. Some of 

 the species are phosphorescent. By far the greater 

 number have a thick, spongj' outer skin, which is often 

 strengthened by having variously-shaped calcareous 

 grains or rugose, and more or less fusiform calcareous 

 spicula, imbedded in its surface. In general the com- 

 mon mass has an expanded base, by which it is attached 

 to some marine body, and when the mass assumes an 

 erect or branched tree-like form, the animals secrete in 

 the centre of their body a more or less rigid support, 

 which has been called their axis, and which has some- 

 times, though erroneously (from its being commonly 

 seen in collections without the remains of tlie investing 

 animal), been considered the entire coral. This axis 

 is thickened by depositions of fresh layers of lioriiy 

 matter on its surface, as the mass increases in size and 

 requires more support ; the increase of the thickness 

 and length of the axis being always simultaneous with 



